Steel pipe excellent in deformation characteristics and method of producing the same

ABSTRACT

The invention provides a steel pipe excellent in deformation characteristics, most notably a steel pipe for expandable-pipe oil well and a low-yield-ratio line pipe, and a method of producing the same without conducting water cooling requiring large-scale heat treatment equipment, namely a method of producing a steel pipe excellent in deformation characteristics whose microstructure is a two-phase structure including a martensite-austenite constituent at an area fraction of 2 to 10% and a soft phase, which method comprises: heating at Ac 1 +10° C. to Ac 1 +60° C. and thereafter cooling a precursor steel pipe which contains, in mass %, C: 0.04 to 0.10% and Mn: 1.00 to 2.50%, is limited to Si: 0.80% or less, P: 0.03% or less, S: 0.01% or less, Al: 0.10% or less and N: 0.01% or less, further contains one or more of Ni: 1.00% or less, Mo: 0.60% or less, Cr: 1.00% or less and Cu: 1.00% or less, where content of Mn and content of one or more of Cr, Ni, Mo and Cu satisfy Mn+Cr+Ni+2Mo+Cu≧2.00, and a balance of iron and unavoidable impurities.

FIELD OF THE INVENTION

This invention relates to a steel pipe excellent in deformationcharacteristics, e.g., an oil-well steel pipe for expandable tubularapplications that is excellent in expansion characteristics and suitablefor use as an expandable oil-well pipe to be expanded after insertioninto the well when drilling an oil well or gas well or anelectric-resistance-welded line pipe with low yield ratio in the pipelongitudinal direction and suitable for a submarine pipeline laid usinga reel barge, a method of producing the same, and a method of producinga precursor steel pipe for the steel pipe excellent in deformationcharacteristics.

DESCRIPTION OF THE RELATED ART

Steel pipe for use in oil wells has conventionally been used by runningit down the well in its form as manufactured. However, recent years haveseen the development of a technology for use at the time of drilling anoil well or gas well that enables a steel pipe to be inserted into thewell and then expanded inside the well (“expandable-pipe oil well”).This technology is making a major contribution to oil well and gas welldevelopment cost reduction.

In the early stage of expandable-pipe oil well development, the steelpipe was expanded about 10% and an ordinary oil-well pipe was used asthe expandable-pipe for the expandable-pipe oil well. However, as higherpipe expansion ratios were applied and came to exceed 20%, increase inthickness unevenness became an issue. In other words, unevenness in thethickness of the expandable pipe used in the expandable-pipe oil wellled to local wall thickness loss that degraded the performance of thesteel pipe and led to fracture and other problems. This placed a limiton the pipe expansion ratio.

The inventors earlier developed steel pipes excellent in expansioncharacteristics that can be used in expandable-pipe oil wells (see, forexample, International Publications WO2005/080621 and WO2006/132441).The steel pipe taught by International Publication WO2005/080621 has atwo-phase structure of fine martensite dispersed in a ferrite structureand is excellent in pipe expandability. The steel pipe having atwo-phase structure is low in yield strength and high in work hardening.As a result, the stress required for pipe expansion is low, so that thesteel pipe has excellent expansion characteristics in the point of notreadily experiencing local contraction.

The steel pipe taught by International Publication WO2006/132441 has acomposition with limited carbon content and a structure of temperedmartensite. It is therefore high in toughness and excellent in expansioncharacteristics. However, these steel pipes having a two-phase structureof fine martensite dispersed in ferrite structure or a temperedmartensite structure are produced by quenching. They therefore requirelarge-scale heat treatment equipment for heating and water cooling thesteel pipe.

Moreover, the design concept of line pipes is changing from one based onstrength standards to one based on strain standards. As a result, lowyield ratio in the longitudinal direction of the pipe has becomenecessary. This is aimed at preventing local buckling when stress occursin the pipeline owing to ground movement after installation. On theother hand, when a pipeline is laid on the sea bottom, the reel bargemethod is used in which the line pipe is once reeled into a coil andthen unreeled onto the seabed. Therefore, in order to avoid bucklingduring the reeling and unreeling, the line pipe needs to have highdeformability, i.e., low yield ratio, in the longitudinal direction.

As the quality of the electric-resistance welds ofelectric-resistance-welded steel pipes has improved in recent years,electric-resistance-welded steel pipes have come into wide use in linepipe applications because they are lower in cost than seamless steelpipes and UO steel pipes. However, an electric-resistance-welded steelpipe generally has high yield ratio since it is used as cold formed fromhot coil. The yield ratio of a steel pipe having a large ratio of wallthickness to outside diameter, such as one used in a submarine pipeline,is especially high because its cold-working strain increases inproportion as the thickness/diameter ratio is greater. And resistancereduction in the pipe longitudinal direction by the Bauschinger effectcannot be expected because the steel pipe experiences substantially nocompressive stress load during formation.

Many techniques for reducing yield ratio in the longitudinal directionof an electric-resistance-welded steel pipe have been developed (see,for example, Japanese Patent Publication (A) No. 2006-299415). Thesetechniques focus on reducing beforehand the yield ratio of the hot coilthat is the steel pipe starting material. Actually, however, the yieldratio of the starting material, no matter how far it is reduced, hassubstantially no effect on the yield ratio of the formed pipe becauseresistance is markedly increased by the work hardening during pipeforming.

Techniques have also been developed that use the Bauschinger effect tolower resistance by imparting compressive strain in the longitudinaldirection during the sizing process after pipe making (see, for example,Japanese Patent Publication (A) No. 2006-289482). In industrialproduction, however, it is extremely difficult to impart longitudinalcompressive strain without buckling the steel pipe.

And while not for application to pipe line, there have also beendeveloped methods for producing low yield ratioelectric-resistance-welded steel pipe for construction use by heattreatment after pipe making (see, for example, Japanese Patent No.3888279). However, these techniques cannot provide the high levelstrength, toughness and weldability required by line pipe.

SUMMARY OF THE INVENTION

As pointed out in the foregoing, conventional steel pipes that rely on atwo-phase structure of fine martensite dispersed in ferrite structure ora tempered martensite structure to achieve excellent deformationcharacteristics require quenching and other heat treatment after pipeforming. This makes large-scale heat treatment equipment necessary. Andwhen production of steel pipe with low yield ratio in the longitudinaldirection and excellent deformation characteristics is attempted by themethod of using a hot coil with low yield ratio or the method ofimparting compressive stress in the pipe longitudinal direction, it isin fact found to be impossible to achieve the desired low yield ratio.Further, while the method of applying heat treatment after pipe makingcan achieve low yield ratio, a technique for securing thecharacteristics required by line pipe is required. Therefore, productionof line pipe having a low yield ratio in the longitudinal direction isdifficult, particularly in the case of electric-resistance-welded steelpipe.

The present invention utilizes simple heat treatment without need forwater cooling requiring large-scale heat treatment equipment to providea steel pipe excellent in deformation characteristics, e.g., an oil-wellsteel pipe for expandable tubular applications that is excellent inexpansion characteristics or a line pipe with low yield ratio in thepipe longitudinal direction, a method of producing the same, and amethod of producing a precursor steel pipe (a pipe before heattreatment) for the steel pipe excellent in deformation characteristics.

For improving deformation characteristics, specifically for enhancingexpansion characteristics and lowering yield ratio, it is effective toincrease the work-hardening coefficient. The inventors thereforeconcluded that it was necessary to adopt as the steel pipe structure atwo-phase structure comprising a soft phase and a hard second phase.When heat treatment is performed for obtaining such a two-phasestructure, water cooling for obtaining the hard phase requires use oflarge-scale heat treatment equipment. So it is desirable for the lowyield ratio to be obtainable even by air cooling. However, owing to thefact that the cooling rate by air cooling is slower than that by watercooling, the portion transformed to austenite during heating of thesteel pipe to the two-phase region decomposes into ferrite and cementiteduring the air cooling, making it difficult to obtain martensite orbainite as a hard second phase.

Upon considering this problem, the inventors figured that if amartensite-austenite constituent (sometimes called MA in the following),which is obtainable even at a relatively slow cooling rate, should beused as the hard second phase, it might be possible to obtain a steelpipe having a two-phase structure with high work hardening even by aircooling. Through studies conducted in line with this conjecture theydiscovered that a two-phase structure comprising a soft phase and a hardsecond phase with a high work-hardening coefficient can be obtained evenby air cooling conducted after heat treatment, provided that thechemical constituents of the steel pipe are regulated to suitable rangesand the heating is performed at an appropriate temperature.

This invention was achieved based on this knowledge and the gist thereofis as set out below.

(1) A steel pipe excellent in deformation characteristics whichcontains, in mass %, C: 0.04 to 0.10% and Mn: 1.00 to 2.50%, is limitedto Si: 0.80% or less, P: 0.03% or less, S: 0.01% or less, Al: 0.10% orless and N: 0.01% or less, further contains one or more of Ni: 1.00% orless, Mo: 0.60% or less, Cr: 1.00% or less and Cu: 1.00% or less, wherecontent of Mn and content of one or more of Cr, Ni, Mo and Cu satisfy

Mn+Cr+Ni+2Mo+Cu≧2.00,

and a balance of iron and unavoidable impurities,

and whose microstructure is a two-phase structure including amartensite-austenite constituent at an area fraction of 2 to 10% and asoft phase.

(2) A steel pipe excellent in deformation characteristics according to(1), wherein the soft phase is one or more of ferrite, high-temperaturetempered martensite, and high-temperature tempered bainite.

(3) A steel pipe excellent in deformation characteristics according to(1) or (2), further containing, in mass %, one or more of Nb: 0.01 to0.30%, Ti: 0.005 to 0.03%, V: 0.30% or less, B: 0.0003 to 0.003%, Ca:0.01% or less, and REM: 0.02% or less.

(4) A steel pipe excellent in deformation characteristics according toany of (1) to (3), wherein a work-hardening coefficient in thecircumferential direction of the steel pipe is 0.10 or greater.

(5) A steel pipe excellent in deformation characteristics according toany of (1) to (4), wherein the steel pipe has a ratio of wall thicknessto outside diameter of 0.03 or greater.

(6) A steel pipe excellent in deformation characteristics according toany of (1) to (5), wherein the wall thickness of the steel pipe is 5 to20 mm.

(7) A steel pipe excellent in deformation characteristics according toany of (1) to (6), wherein the outside diameter of the steel pipe is 114to 610 mm.

(8) A steel oil-well pipe for expandable-pipe oil well comprising asteel pipe excellent in deformation characteristics according to any of(1) to (7), which steel oil pipe for expandable-pipe oil well is to beexpanded in a well and whose steel pipe has a wall thickness of 5 to 15mm and an outside diameter of 114 to 331 mm.

(9) A line pipe comprising a steel pipe excellent in deformationcharacteristics according to any of (1) to (7), whose steel pipe has awall thickness of 5 to 20 mm and an outside diameter of 114 to 610 mm.

(10) A method of producing a steel pipe excellent in deformationcharacteristics whose microstructure is a two-phase structure includinga martensite-austenite constituent at an area fraction of 2 to 10% and asoft phase, which method comprises:

heating at Ac₁+10° C. to Ac₁+60° C. and thereafter cooling a precursorsteel pipe which contains, in mass %, C: 0.04 to 0.10% and Mn: 1.00 to2.50%, is limited to Si: 0.80% or less, P: 0.03% or less, S: 0.01% orless, Al: 0.10% or less and N: 0.01% or less, further contains one ormore of Ni: 1.00% or less, Mo: 0.60% or less, Cr: 1.00% or less and Cu:1.00% or less, where content of Mn and content of one or more of Cr, Ni,Mo and Cu satisfy

Mn+Cr+Ni+2Mo+Cu≧2.00,

and a balance of iron and unavoidable impurities.

(11) A method of producing a steel pipe excellent in deformationcharacteristics according to (10), wherein the precursor steel pipefurther contains in mass %, one or more of Nb: 0.01 to 0.30%, Ti: 0.0.05to 0.03%, V: 0.30% or less, B: 0.0003 to 0.003%, Ca: 0.01% or less, andREM: 0.02% or less.

(12) A method of producing a precursor steel pipe for a steel pipeexcellent in deformation characteristics according to claim 10 or 11,wherein the method further comprising the steps of:

heating at 1000° C. to 1270° C. a slab containing the chemicalcompositions claimed in claim 10 or 11,

hot rolling the heated slab to a finish rolling reduction of 50% orgreater;

forming the obtained steel plate into an open pipe shape; and

welding the seam.

(13) A method of producing a precursor steel pipe for a steel pipeexcellent in deformation characteristics according to (12), wherein theslab further contains, in mass %, one or more of Nb: 0.01 to 0.30%, Ti:0.005 to 0.03%, V: 0.30% or less, B: 0.0003 to 0.003%, Ca: 0.01% orless, and REM: 0.02% or less.

The present invention enables production of a steel pipe excellent indeformation characteristics, e.g., an oil well steel pipe for expandabletubular applications that is excellent in expansion characteristics or aline pipe with low yield ratio, utilizing air cooling after pipeheating, without need for large-scale heat treatment equipment forheating and water cooling the steel pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing how the MA content of the air-cooled steelpipe varies with the amount of added Mn, Ca, Ni, Mo and Cu.

DETAILED DESCRIPTION OF THE INVENTION

The inventors conducted a study on methods for producing steel pipesthat have a two-phase structure comprising a soft phase and a hardsecond phase and are excellent deformation characteristics, withparticular focus on methods for producing high-strength steel pipeexcellent in expansion characteristics and line pipe with low yieldratio by air cooling a steel pipe after heating it throughout.

When a steel increased in hardenability and containing elements notreadily soluble in cementite is heated to the two-phase region betweenthe Ac₁ transformation temperature and the Ac₃ transformationtemperature, the austenite formed tends during air cooling to become MA(martensite-austenite constituent), without decomposing into carbide andferrite. Elements that have this effect include Mn, Cr, Ni, Mo and Cu.

The inventors therefore investigated how the amount of MA formed afterheating to the two-phase region and air cooling varies with the amountof added Mn, Cr, Ni, Mo and Cu. Specifically, they produced steel platesby incorporating various amounts of Ni, Mo, Cr and Cu into steel with abasic composition of, in mass %, C: 0.04 to 0.10%, Mn: 1.40 to 2.50%,Si: 0.80% or less, P: 0.03% or less, S: 0.01% or less, Al: 0.10% or lessand N: 0.01% or less. The plates were heat treated by heating to 700 to800° C. and air cooling.

Samples for microstructure observation were taken from the plates afterheat treatment, leveled by etching, and observed with a lightmicroscope. The structures were photographed. The white colored regionsin the microstructure photographs were identified as MA and the areafractions of the regions were determined by image analysis. Specimenstaken from the plates were tensile-tested, a log-log graph of truestrain vs true stress was prepared, and the work-hardening coefficient(n value) was determined from the slope of the linear section. Thetensile strengths of the plates were between 600 and 800 MPa.

It was found that when the heating temperature is less than Ac₁+10° C.,the n value becomes less than 0.1. This is because only a small amountof austenite is formed during the heating so that the amount of MAformed after air cooling is also small. On the other hand, althoughheating at a temperature greater than Ac₁+60° C. increases the amount ofaustenite formed, the amount of C distributed in the austenitedecreases. The austenite therefore becomes unstable and decomposes intoferrite and cementite during air cooling. As a result, the MA areafraction declines, so that, as in the case of heating at a lowtemperature, the n value becomes less than 0.1.

These findings prompted the inventors to analyze how the MA content of asteel pipe air cooled after heating in the temperature range of Ac₁+10°C. to Ac₁+60° C. varies with the amount of Mn, Cr, Ni, Mo and Cuaddition. As shown in FIG. 1, the analysis showed that the MA contentcan be correlated to Mn+Cr+Ni+2Mo+Cu as an index. When any of Cr, Ni, Moand Cu is intentionally omitted from among the selected elements,Mn+Cr+Ni+2Mo+Cu is calculated using 0 as the value of the omittedelement or elements.

Ac₁ is calculated based on the contents (mass %) of Si, Mn, Ni and Cramong the steel constituents as:

Ac₁=723+29.1×Si−10.7×Mn−16.9×(Ni—Cr).

When any among the deoxidizing element Si and the optional elements Niand Cr are intentionally omitted, Ac₁ is calculated using 0 as the valueof the omitted element or elements.

In the graph of FIG. 1, “MA” scaled on the vertical axis is the areafraction of MA. As will be understood from the explanation that follows,the MA area fraction becomes 2% or greater when Mn+Cr+Ni+2Mo+Cu is 2.00or greater. The MA area fraction increases with increasing value ofMn+Cr+Ni+2Mo+Cu. The reason for this is thought to be that the improvingstability of austenite with increasing value of Mn+Cr+Ni+2Mo+Cuincreases the amount remaining as MA after air cooling.

The inventors further produced hot-rolled steel plates based on steelplate chemical compositions for establishing MA area fractions of 2 to10% and work-hardening coefficients of 0.10 or greater. The plates wereformed into electric-resistance-welded steel pipes. Each steel pipe washeated in the temperature range of Ac₁+20° C. to Ac₁+60° C., air cooled,and expanded by forcing a pipe expansion plug into one end, whereafterthe limit pipe expansion ratio up to which no cracking occurred wasdetermined. Specimens taken along the circumference of the pipes weretensile-tested to determine their work-hardening coefficients. The limitpipe expansion ratio was found to be 20% or greater when thework-hardening coefficient was 0.10 or greater and to be 30% or greaterwhen the work-hardening coefficient was 0.15 or greater.

Similarly, steel plates were produced by incorporating various amountsof Ni, Mo, Cr and Cu into steel with a basic composition of, in mass %,C: 0.04 to 0.10%, Mn: 1.00 to 2.50%, Si: 0.80% or less, P: 0.030% orless, S: 0.010% or less, Al: 0.10% or less and N: 0.010% or less. Theplates were pre-strained at a rate equivalent to pipe forming strain of4% and heat treated by heating to 700 to 800° C. and air cooling.Samples for microstructure observation taken from the plates after heattreatment were observed with a light microscope and their MA areafractions were determined by image analysis.

The yield ratio of the pre-strained steel plates was 0.92. MA formationafter air cooling was low when the heating temperature was less thanAc₁+10° C. When it exceeded Ac₁+60° C., the austenite decomposed intoferrite and cementite during air cooling. In either case, the MA areafraction declined and the yield ratio decreased only to around 0.90.

A total of 27 types of steel varied in composition within the ranges ofMn: 1.00 to 2.5%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 0.6% and Cu: 0to 1.0% were prepared, heated in the temperature range of Ac₁+10° C. toAc₁+60° C., and variation of the MA content of air-cooled, pre-strainedsteel plates with the amounts of added Mn, Cr, Ni, Mo and Cu wasanalyzed. Multiple regression analysis of the results showed that thebest correlation with MA content is obtained when Mn+Cr+Ni+2Mo+Cu isused as an index.

That is, it was found that, as seen in FIG. 1, the amount of MA can becorrelated to Mn+Cr+Ni+2Mo+Cu as an index. All heating temperatureswithin the temperature range of Ac₁+10° C. to Ac₁+60° C. gave resultssimilar to those in FIG. 1. The inventors further produced hot-rolledsteel plate using steels of compositions for achieving MA area fractionsof 2 to 10% by the aforesaid heat treatment and formed them into pipeshaving a ratio of wall thickness to outside diameter of 0.05. The pipeswere heated, air cooled, and specimens taken along the circumference ofthe pipes were tensile-tested to determine their work-hardeningcoefficients. It was found that when the heating temperature is in therange of Ac₁+10° C. to Ac₁+60° C., MA is 2% or greater, so that yieldratio falls to 0.90 or less.

The chemical composition of the steel pipe excellent in deformationcharacteristics according to the present invention and the reasons forlimiting the constituents thereof are explained in the following. Thechemical composition of the invention steel pipe is defined within thefollowing ranges from both the viewpoint of the structure and strengthof the steel plate before pipe making and the viewpoint of the structureand strength of the pipe after heat treatment.

C stabilizes austenite during heating at Ac₁+10° C. to Ac₁+60° C.,preferably Ac₁+20° C. to Ac₁+60° C., and as such is an extremelyimportant element in the present invention for increasing MA areafraction after air cooling. C must be added to a content of 0.04% orgreater to secure the desired amount of MA after heat treatment. C isalso an element that improves steel strength by enhancing hardenability.Since excessive addition degrades toughness by increasing strength toomuch, the upper limit of C addition is defined as 0.10%. The morepreferable upper limit of C content is less than 0.10%.

Mn is an indispensable element for increasing hardenability and securinghigh strength. It is also an element that stabilizes austenite bylowering the Ac₁ point. However, Mn must be added to a content of 1.00%or greater so that MA decomposition after air cooling can be inhibitedby forming austenite during heating at Ac₁+10° C. to Ac₁+60° C.,preferably Ac₁+20° C. to Ac₁+60° C. The lower limit of Mn content ispreferably 1.40% or greater. However, when the Mn content is too high,the martensite content of the steel plate from which the steel pipe isfabricated becomes excessive. As this degrades formability by makingstrength too high, the upper limit of Mn content is defined as 2.50%.

Si is a deoxidizing element that markedly degrades low-temperaturetoughness when a large amount is added. The upper limit of Si content istherefore defined as 0.80%. In this invention, Al and Ti can also beused as steel deoxidizers, so addition of Si is not absolutelynecessary. However, as Si improves strength and promotes MA formation,it is added to a content of 0.10% or greater.

P and S are impurities whose upper limits are defined as 0.03% and0.01%, respectively. P content reduction alleviates center segregationof the continuously cast slab, thereby preventing intergranular fractureand improving toughness. S content reduction works to improve ductilityand toughness by reducing MnS that elongates during hot rolling.

Al is a deoxidizing element. When added to a content of greater than0.10%, it increases nonmetallic inclusions that impair the steelcleanliness. The upper limit of Al content is therefore defined as0.10%. When Ti and Si are used as deoxidizers, Al addition is notabsolutely necessary, so no lower limit of Al content is defined. But Alis usually present as an impurity at a content of 0.001% or greater.When AlN is utilized for steel structure refinement, Al is preferablyadded to a content of 0.01% or greater.

N is an impurity whose upper limit is defined as 0.01% or less. In thecase of optionally adding Ti, if N is incorporated at a content of0.001% or greater, TiN forms to suppress austenite grain coarseningduring slab reheating, thereby improving base material toughness. Butwhen N content exceeds 0.01%, TiN coarsens and gives rise to surfaceflaws, toughness degradation and other adverse effects.

When, as mentioned above, one or more of Ni, Mo, Cr and Cu are added inaddition to the required element Mn so as to satisfy

Mn+Cr+Ni+2Mo+Cu≧2.00,

the desired amount of MA can be secured because austenite does notreadily decompose into ferrite and cementite during air cooling. In theexpression above, Mn, Cr, Ni, Mo and Cu represent the contents (mass %)of the respective elements. When addition of any of the optionalelements Cr, Ni, Mo and Cu is intentionally omitted, the left side ofthe expression is calculated using 0 as the value of the omitted elementor elements.

Moreover, Ni, Mo, Cr and Cu are elements that improve hardenability, sothat one or more of them are preferably added to realize high strength.

Ni is also effective for finely forming austenite during heating of thesteel in the two-phase region. But addition of too much Ni makes themartensite content of the steel plate from which the pipe is fabricatedexcessive. As this degrades formability by making strength too high, theupper limit of Ni content is preferably defined as 1.00%.

When Mo, Cr and Cu are excessively added, hardenability increases. Asthis may degrade formability by making the strength of the steel platefrom which the pipe is fabricated too high, the upper limits of Mo, Crand Cu addition are preferably defined as 0.60%, 1.00% and 1.00%,respectively.

One or more of Nb, Ti, V, B, Ca and REM can be further added. Nb, Ti andV contribute to steel structure refinement, B contributes tohardenability improvement, and Ca and REM contribute to inclusionmorphology control.

Nb is an element that inhibits recrystallization of austenite duringrolling. Addition of Nb to a content of 0.01% or greater is preferablefor refining the grain diameter of the steel before heating. Nb is alsopreferably added for securing the toughness required by a line pipe. Butaddition of Nb in excess of 0.30% degrades toughness, so the upper limitof addition is preferably defined as 0.30%.

Ti is an element that forms fine TiN, thereby inhibiting austenite graincoarsening during slab reheating. And when Al content low, e.g., 0.005%or less, Ti functions as a deoxidizer.

For improving toughness by refining the microstructure utilizing Tiaddition, N is preferably incorporated at a content of 0.001% or greaterand Ti added to a content of 0.005% or greater. But when the Ti contentis too great, toughness deteriorates owing to TiN coarsening and/orTiC-induced precipitation hardening. The upper limit of Ti addition istherefore preferably defined as 0.03%.

V has substantially the same effects as Nb but at a somewhat weakerlevel. For enabling V to exhibit its effects, the element is preferablyadded to a content of 0.01% or greater. But as excessive V additiondegrades toughness, the upper limit of V addition is preferably definedas 0.30%.

B is an element that increases steel hardenability and promotes MAformation by inhibiting decomposition of austenite into ferrite andcarbide during air cooling. For obtaining these effects, B is preferablyadded to a content of 0.0003% or greater. However, addition of B inexcess of 0.003% may cause loss of toughness owing to formation ofcoarse B-containing carbides. The upper limit of B addition is thereforepreferably defined as 0.003%.

Ca and REM are elements that contribute to toughness improvement bycontrolling formation of MnS and other sulfides. Addition of either orboth is therefore preferable. To obtain this effect, Ca is preferablyadded to a content of 0.001% or greater and REM to a content of 0.002%or greater. However, when Ca addition exceeds 0.01% or REM additionexceeds 0.02%, the cleanliness of the steel may be impaired by formationof large clusters and large inclusions as a result of the generation ofCao—CaS or REM-CaS. It is therefore preferable to set the upper limit ofCa addition at 0.01% and the upper limit of REM addition at 0.02%. Thestill more preferable upper limit of Ca addition is 0.006%.

The structure of the heat-treated steel pipe will be explained next.

In order to achieve excellent deformation characteristics, particularlyto improve pipe expandability and lower yield ratio, the steel pipe ispreferably given a two-phase structure comprising, in terms of areafraction, 2 to 10% of MA and the balance of soft phase. On the otherhand, when the austenitic ratio increases to 10% or greater duringheating in the two-phase region, the austenite decomposes into ferriteand cementite during air cooling because its C concentration becomesinsufficient. Realizing MA of greater than 10% is therefore difficult.

After leveling by etching, MA looks white when observed with a lightmicroscope. Moreover, when a sample subjected to nital etching isobserved with a scanning electron microscope (SEM), the MA, which isresistant to the etching, is observed as a structure present in the formof flat islands. The MA area fraction can therefore be measured by imageanalysis of a light microscope structure micrograph of a sample leveledby etching or of an SEM structure micrograph of the structure of a nitaletched sample.

Deformation characteristics, particularly pipe expandability, improve aswork hardening is easier. Excellent pipe expandability can therefore beachieved by establishing an MA area fraction of 2 to 10% so as to makethe work-hardening coefficient in the circumferential direction of thesteel pipe of 0.10 or greater.

Portions other than the MA are soft phases, namely, the ferrite,martensite and bainite structures of the steel pipe before heattreatment after being heated at Ac₁+10° C. to Ac₁+60° C., preferablyAc₁+20° C. to Ac₁60° C., and then air cooled.

In the present invention, the martensite and bainite softened by heatingto Ac₁+10° C. to Ac₁+60° C., preferably Ac₁+20° C. to Ac₁+60° C., andair cooling are high-temperature tempered martensite andhigh-temperature tempered bainite. In other words, the soft phase is oneor more ferrite, high-temperature tempered martensite, andhigh-temperature tempered bainite.

Ac₁ of a steel in the composition range of the invention can becalculated using the following equation:

Ac₁=723+29.1×Si−10.7×Mn−16.9×(Ni—Cr),

where Si, Mn, Ni and Cr represent the contents (mass %) of theassociated elements.

Ac₁ can also be determined experimentally using a specimen taken fromthe produced steel plate or a steel of the same composition produced inthe laboratory. For example, the transformation temperature during steelheating can be determined by the so-called Formaster test of heating thetest piece at a constant rate and measuring expansion.

The austenite transformation start temperature (Ac₁) and austenitetransformation end temperature (Ac₃) can be found by determining thetemperatures of the start and end points of the bend from therelationship between temperature and expansion ascertained by theFormaster test.

Ordinarily, when a steel is heated from Ac₁ to Ac₃, some martensite,bainite and ferrite transforms to austenite and the remainderprogressively recovers to body-centered cubic structure as is.

Of particular note regarding the production method of the presentinvention is that since the heating is performed in the relatively lowtemperature zone of Ac₁+10° C. to Ac₁+60° C., preferably Ac₁+20° C. toAc₁+60° C., much of the martensite and bainite present before theheating does not transform to austenite but remains as soft phase asthough having undergone tempering. In other words, when the martensiteand bainite formed in the steel pipe before heat treatment is heated atAc₁+10° C. to Ac₁+60° C., preferably Ac₁+20° C. to Ac₁+60° C., theysoften owing to dislocation recovery and precipitation of solute C,thereby becoming high-temperature tempered martensite andhigh-temperature tempered bainite.

The ferrite includes some that was also ferrite before heating andprogressively recovered during heating, and some that transformed toaustenite during heating at Ac₁+10° C. to Ac₁+60° C., preferably Ac₁+20°C. to Ac₁+60° C., and then reverse-transformed during air cooling. Inother words, ferrite and cementite from ferrite decomposition are mixedtogether. However, the two are collectively called ferrite because it isdifficult to distinguish them with a light microscope.

The steel pipe excellent in deformation characteristics of the presentinvention having the aforesaid composition and metallurgical structurehas a tensile strength of 500 to 900 MPa and a wall thickness of 5 mm to20 mm. Particularly in the case of a steel pipe for expandable-pipe oilwell, tensile strength of 550 to 900 MPa and thickness of 5 mm to 15 mm,preferably 7 mm to 15 mm are required. In a low-yield-ratio line pipe,tensile strength of 500 to 750 MPa and thickness of 5 mm to 20 mm arerequired.

The production conditions for the steel pipe excellent in deformationcharacteristics having the aforesaid composition will be explained next.The method of producing the steel pipe excellent in deformationcharacteristics of the present invention consists in heat treating aprecursor steel pipe without subjecting it to diameter-reduction rollingor other hot working. However, sizing for roundness improvement or coldworking for shape correction can be conducted before the heat treatment.

The method of producing the steel pipe excellent in deformationcharacteristics according to the present invention is basicallycharacterized by the production conditions explained in the foregoing,namely, by heat treating a precursor steel pipe at Ac₁+10° C. to Ac₁+60°C., preferably Ac₁+20° C. to Ac₁+60° C. and subsequent air cooling. Thepresent invention therefore enables improvement of deformationcharacteristics solely by air cooling after heat treating the precursorsteel pipe throughout, thus eliminating the need for water coolingrequiring large-scale heat treatment equipment. Worth noting is thatwater cooling after the heat treatment produces martensite, not MA. Thepipe heat treatment temperature is specified as Ac₁+10° C. to Ac₁+60°C., preferably Ac₁+20° C. to Ac₁+60° C., in order to form MA after aircooling. This formation of MA occurs because when partial transformationto austenite occurs upon heating to the two-phase region, C concentratesin the austenite, with substantially no distribution of other elementstherein.

More specifically, when the heating temperature is less than Ac₁+10° C.,the low rate of transformation to austenite makes it difficult to obtainthe desired amount of MA. For increasing the amount of austenite formedduring heating, the heating temperature is preferably made Ac₁+20° C. orgreater. On the other hand, when heating is performed at a temperatureexceeding Ac₁+60° C., the amount of transformation to austenite becomestoo great. As this makes the C concentration in the austenitic phaseinsufficient, the air cooling decomposes the austenite into ferrite andcementite, making it hard to obtain enough MA. Further, the upper limitof the heating temperature is preferably defined as 780° C. or less toensure fine grain diameter. The chemical composition of the steel pipeis therefore preferably defined so that Ac₁ is 720° C. or less.

Although any production method can be used to produce the inventionsteel pipe excellent in deformation characteristics (e.g., steel pipefor expandable-pipe oil well or low-yield-ratio line pipe), a methodthat minimizes thickness unevenness is preferable. If thicknessunevenness is small, even a seamless pipe suffices. However, a weldedpipe is generally fabricated by butt-welding a steel plate hot rolled tohigh plate thickness accuracy and is therefore lower in thicknessunevenness than a seamless pipe.

As the method for forming the welded pipe, it suffices to adopt anordinarily used pipe forming method, namely, press forming or rollforming. Although laser welding, arc welding orelectric-resistance-welding can be used as the butt-welding method, thehigh productivity of the electric resistance welding process makes itespecially suitable for fabricating the invention steel pipe, andparticularly for the invention oil well pipe and line pipe.

The hot-rolled plate is obtained by hot rolling a steel slab in theaustenite region, then conducting rough rolling followed by finishrolling. Forced cooling is preferably performed after the finishrolling. The steel plate that is the starting material preferably has atensile strength of 600 to 800 MPa.

The hot rolling temperature is preferably made 1000° C. or greater toensure that the slab assumes an austenitic structure with good hotworkability. But at a hot rolling temperature greater than 1270° C., thestructure coarsens to impair hot workability. The upper limit of the hotrolling temperature is therefore preferably defined as 1270° C.

The finish rolling is preferably performed at a reduction of 50% orgreater in order to refine the grain diameter of the pipe. The finishrolling reduction is determined by dividing the difference in platethickness between before and after rolling by the plate thickness beforerolling. When the finish rolling is conducted at a reduction of 50% orgreater, austenite is formed and dispersed uniformly during heating ofthe steel pipe in the two-phase region, thereby finely dispersing MA andimproving the pipe expansion characteristics.

Forced cooling performed following finish rolling gives the hot-rolledplate a multiphase structure including ferrite, martensite, and bainite.(The most common multiphase structure is one of ferrite and bainite.)The desired multiphase structure can be established by, for example,following the finish rolling with cooling at 15° C./s and coiling at 400to 500° C. This ensures even more uniform dispersion of austenite duringheating of the pipe in the two-phase region. As MA therefore dispersesfinely, the deformation characteristics are enhanced, with particularimprovement of expansion characteristics and reduction of yield ratio.

Among the steel pipes excellent in deformation characteristics obtainedby the invention production method, the steel pipe for expandable oilwell can be inserted into a well drilled in the ground using a drillpipe or into a well in which another oil well pipe is already installed.Wells sometimes reach a depth of several thousand meters. The steel pipefor expandable oil well that is expanded inside the well preferably hasa wall thickness of 5 to 15 mm and outside diameter of 114 to 331 mm.

In the case of laying the low-yield-ratio line pipe obtained by theinvention production method on the seabed to build a submarine pipeline,it is possible to employ the reel barge method. The line pipe ispreferably an electric-resistance-welded pipe, and preferably has a wallthickness of 5 to 20 mm and an outside diameter of 114 to 610 mm.

Example 1

Steels containing the chemical compositions shown in Table 1 wereproduced in a converter and continuously cast into slabs. Each of theobtained slabs was heated to 1100 to 1200° C., rolled with a continuoushot-rolling mill at a reduction of 70% or greater, cooled at 10 to 20°C./s, and coiled at 400 to 500° C. to produce a 9.56-mm thick hot-rolledplate.

The hot-rolled plate was used as a material for producing a steel pipeof 193.7-mm outside diameter by the electric resistance pipe weldingprocess. The obtained pipe was heated for 120 s at the temperature shownin Table 2 and then subjected to air-cooling heat treatment. The symbol“0” in Table 1 means that addition of the optional element wasintentionally omitted.

A specimen taken along the circumference of the pipe was tensile-testedto determine yield strength (YS), tensile strength (TS) andwork-hardening coefficient (n value). A log-log graph of true strain vstrue stress was prepared, and the n value was determined from the slopeof the linear section. A pipe expansion test of expanding the pipe 30%was conducted at one end of the pipe using a plug. After the expansion,the wall thickness distribution of the expanded pipe was measured. Thedifference relative to the average wall thickness was calculated and themaximum wall thickness loss value was used an index of maximum wallthickness loss.

The structure of the steel pipe was observed with a light microscope.The area fraction of the MA was determined by image analysis of amicrostructure photograph of a sample that had been leveled by etching.The portion other than MA consisted of ferrite, martensite and bainite.Vickers hardness measurement confirmed that martensite and bainite hadsoftened.

The results are shown in Table 2. In Table 2, YS/TS (yield ratio) is theratio of yield strength to tensile strength (Y/T) expressed in percent.It is clear from Table 2 that the invention steel pipes experienced onlysmall maximum wall thickness loss on the order of around 0.6 mm or lessand exhibited excellent expansion characteristics equal to or betterthan Example No. 7 in which water cooling was performed. Note thatExample No. 7 is a Comparative Example that did not satisfyMn+Cr+Ni+2Mo+Cu≧2.00 and used water cooling. The symbol (9) indicatedfor Example No. 7 in the MA area fraction column means that the areafraction of martensite formed during water cooling after pipe heatingwas 9%.

In Example No. 6 the heating temperature was too high and in Example No.8, as in Example No. 7, the steel composition was outside the rangesspecified by the present invention. In these Examples, MA formationafter air cooling was insufficient and large wall thickness loss ofgreater than 1 mm occurred.

TABLE 1 Chemical composition (Mass %) Steel C Si Mn P S Al N Ni Mo Cr CuNb A 0.06 0.24 1.84 0.014 0.002 0.03 0.004 0.18 0.11 0.02 0.32 0.04 B0.08 0.12 1.56 0.007 0.001 0.05 0.005 0.48 0.08 0 0 0 C 0.05 0.45 1.610.009 0.002 0.04 0.002 0.17 0 0.15 0.31 0.02 D 0.06 0.08 1.94 0.0120.003 0.03 0.003 0 0.16 0 0 0 E 0.07 0.11 2.01 0.016 0.002 0.06 0.003 00 0.32 0 0.03 F 0.07 0.21 1.45 0.011 0.003 0.02 0.004 0.11 0 0 0 0.04Chemical composition (Mass %) Mn + Cr + Ni + Ac₁ Steel V Ti B Ca REM2Mo + Cu ° C. Remark A 0.05 0.015 0 0 0 2.58 708 Invention B 0 0 0 0.0030 2.2  702 C 0 0.012 0 0 0 2.24 719 D 0.03 0.014 0.0012 0 0 2.26 705 E 00.008 0 0 0.008 2.33 710 F 0 0.016 0 0 0 1.56 712 Comparative Underlinedvalue is outside scope of invention. Ac₁ = 723 + 29.1 × Si − 10.7 × Mn −16.9 × (Ni − Cr)

TABLE 2 Heat MA area Max Example Ac₁ + Ac₁ + temp Cooling fraction YS TSY/T thickness No. Steel 10° C. 60° C. ° C. method % MPa MPa % N valueloss mm Remark 1 A 718 768 760 Air 5 565 778 72.6 0.14 0.61 Invention 2B 712 762 750 Air 6 491 760 64.6 0.16 0.52 3 C 729 779 750 Air 7 505 75866.6 0.17 0.48 4 D 715 765 750 Air 5 499 698 71.5 0.13 0.55 5 E 720 770760 Air 4 512 714 71.7 0.13 0.59 6 A 718 768 820 Air 1 571 667 85.6 0.081.21 Comparative 7 F 722 772 760 Water (9) 433 640 67.7 0.15 0.55 8 F722 772 760 Air 0 450 522 86.2 0.09 1.16 Underlined items are outsidescope of invention. In Example No. 7, value in parentheses in areafraction column is martensite area fraction.

Example 2

Steels containing the chemical compositions shown in Table 3 wereproduced in a converter and continuously cast into slabs. Each of theobtained slabs was heated to 1100 to 1200° C., rolled with a continuoushot-rolling mill at a reduction of 70% or greater, cooled at 10 to 20°C./s, and coiled at 500 to 600° C. to produce a 16-mm or 8 mm thickhot-rolled plate. The hot-rolled plate was used as a material forproducing a steel pipe of 400-mm outside diameter by the electricresistance pipe welding process. A specimen taken from the pipe prior toheat treatment was tensile-tested and the yield ratio (Y/T) wasevaluated.

The obtained pipe was heated for 120 s at the temperature shown in Table4 and then subjected to air-cooling heat treatment. The symbol “0”appearing in an element column of Table 3 means that addition of theoptional element was intentionally omitted. A specimen taken along thelength of the pipe was tensile-tested to determine yield strength (YS)and tensile strength (TS). Toughness was evaluated from ductile-brittletransition temperature (Trs) determined by Vickers impact testing.

The structure of the steel pipe was observed with a light microscope.The area fraction of the MA was determined by image analysis of amicrostructure photograph of a sample that had been leveled by etching.The portion other than MA consisted of ferrite, martensite and bainite.Vickers hardness measurement confirmed that martensite and bainite hadsoftened.

The results are shown in Table 4. In Table 4, YS/TS (yield ratio) is theratio of yield strength to tensile strength (Y/T). It is clear fromTable 4 that all of the invention steel pipes of Example Nos. 11 to 20had post-heat-treatment yield ratios of 0.9 or less, a level enablingapplication in the reel barge method. When the wall thickness to outsidediameter ratio was low, as in Example No. 20, work hardening during pipemaking was low and yield ratio before heat treatment was also low.

Example Nos. 21 to 24 are Comparative Examples. In Example No. 21, theheating temperature was too high, and in Example No. 22, the heatingtemperature was too low. In these Examples, the decrease in yield ratiowas inadequate owing to insufficient MA formation. Example Nos. 23 and24 did not satisfy Mn+Cr+Ni+2Mo+Cu≧2.00, so that hardenability wasinadequate. Although low yield ratio was achieved with water cooling,yield ratio did not decrease sufficiently with air cooling. The symbol(8.0) indicated for Example No. 23 in the MA area fraction column meansthat the area fraction of martensite was 8.0%.

TABLE 3 Chemical composition (Mass %) Steel C Si Mn P S Al N Ni Mo Cr CuNb AA 0.06 0.21 1.76 0.013 0.002 0.03 0.005 0.31 0.09 0.03 0.30 0.05 AB0.08 0.15 1.58 0.007 0.001 0.04 0.005 0.48 0.08 0 0 0.03 AC 0.05 0.431.63 0.009 0.002 0.03 0.003 0.17 0 0.15 0.31 0.03 AD 0.06 0.08 1.930.011 0.003 0.04 0.003 0 0.16 0 0 0.04 AE 0.07 0.12 2.01 0.014 0.0030.06 0.003 0 0 0.32 0 0.03 AF 0.08 0.23 1.18 0.009 0.002 0.04 0.002 0.230.24 0 0.32 0.05 AG 0.07 0.21 2.20 0.015 0.001 0.03 0.003 0 0 0 0 0.03AH 0.07 0.21 1.45 0.011 0.003 0.02 0.004 0.11 0 0 0 0.04 Chemicalcomposition (Mass %) Mn + Cr + Ni + Ac₁ Steel V Ti B Ca REM 2Mo + Cu °C. Remark AA 0.07 0.013 0 0 0 2.58 706 Invention AB 0 0 0 0.003 0 2.22702 AC 0 0.012 0 0 0 2.26 718 AD 0.03 0.014 0 0 0 2.25 705 AE 0 0.010 00 0.008 2.33 710 AF 0.06 0.012 0 0 0 2.21 713 AG 0 0.012 0 0 0 2.20 706AH 0 0.016 0 0 0 1.56 712 Comparative Underlined value is outside scopeof invention. Ac₁ = 723 + 29.1 × Si − 10.7 × Mn − 16.9 × (Ni − Cr)

TABLE 4 Before Thickness/ heat Heat MA area Example Outside treatmentAc₁ + Ac₁ + temp Cooling fraction YS TS Y/T Trs No. Steel diameter Y/T(%) 10° C. 60° C. ° C. method % MPa MPa ° C. ° C. Remark 11 AA 0.04 95716 766 740 Air 4.5 555 778 71.3 −50 Invention 12 AA 0.04 95 716 766 730Air 4.0 545 765 71.2 −50 13 AA 0.04 95 716 766 760 Air 5.5 531 781 68.0−50 14 AB 0.04 96 712 762 740 Air 3.5 482 762 63.3 −55 15 AC 0.04 94 728778 740 Air 3.5 501 757 66.2 −50 16 AD 0.04 96 715 765 740 Air 4.0 487685 71.1 −50 17 AE 0.04 95 720 770 750 Air 4.0 514 711 72.3 −50 18 AF0.04 96 723 773 740 Air 3.0 525 739 71.0 −50 19 AG 0.04 96 716 766 740Air 3.5 534 742 72.0 −50 20 AA 0.02 82 716 766 740 Air 5.0 568 782 72.6−50 21 AA 0.04 95 716 766 790 Air 1.0 608 712 85.4 −55 Comparative 22 AA0.04 95 716 766 700 Air 0 645 715 90.2 −65 23 AH 0.04 96 722 772 750Water (8.0) 434 622 69.8 −50 24 AH 0.04 96 722 772 750 Air 0 454 53385.2 −50 Underlined items are outside scope of invention. In Example No.23, value in parentheses in area fraction column is martensite areafraction.

INDUSTRIAL APPLICABILITY

As set forth in the foregoing, the present invention enables low-costproduction of a steel pipe excellent in deformation characteristics,most notably of a steel pipe for expandable-pipe oil well and alow-yield-ratio line pipe. The present invention therefore makes a veryconsiderable contribution to industry.

1. A steel pipe excellent in deformation characteristics which contains,in mass % C: 0.04 to 0.10% and Mn: 1.00 to 2.50%, is limited to Si:0.80% or less, P: 0.03% or less, S: 0.01% or less, Al: 0.10% or less,and N: 0.01% or less, further contains one or more of Ni: 1.00% or less,Mo: 0.60% or less, Cr: 1.00% or less, and Cu: 1.00% or less, wherecontent of Mn and content of one or more of Cr, Ni, Mo and Cu satisfyMn+Cr+Ni+2Mo+Cu≧2.00, and a balance of iron and unavoidable impurities,and whose microstructure is a two-phase structure including amartensite-austenite constituent at an area fraction of 2 to 10% and asoft phase.
 2. A steel pipe excellent in deformation characteristicsaccording to claim 1, wherein the soft phase is one or more of ferrite,high-temperature tempered martensite, and high-temperature temperedbainite.
 3. A steel pipe excellent in deformation characteristicsaccording to claim 1, further containing, in mass %, one or more of Nb:0.01 to 0.30%, Ti: 0.005 to 0.03%, V: 0.30% or less, B: 0.0003 to0.003%, Ca: 0.01% or less, and REM: 0.02% or less.
 4. A steel pipeexcellent in deformation characteristics according to claim 1, wherein awork-hardening coefficient in the circumferential direction of the steelpipe is 0.10 or greater.
 5. A steel pipe excellent in deformationcharacteristics according to claim 1, wherein the steel pipe has a ratioof wall thickness to outside diameter of 0.03 or greater.
 6. A steelpipe excellent in deformation characteristics according to claim 1,wherein the wall thickness of the steel pipe is 5 to 20 mm.
 7. A steelpipe excellent in deformation characteristics according to claim 1,wherein the outside diameter of the steel pipe is 114 to 610 mm.
 8. Asteel oil-well pipe for expandable-pipe oil well comprising a steel pipeexcellent in deformation characteristics according to claim 1, whichsteel oil pipe for expandable-pipe oil well is to be expanded in a welland whose steel pipe has a wall thickness of 5 to 15 mm and an outsidediameter of 114 to 331 mm.
 9. A line pipe comprising a steel pipeexcellent in deformation characteristics according to claim 1, whosesteel pipe has a wall thickness of 5 to 20 mm and an outside diameter of114 to 610 mm.
 10. A method of producing a steel pipe excellent indeformation characteristics whose microstructure is a two-phasestructure including a martensite-austenite constituent at an areafraction of 2 to 10% and a soft phase, which method comprises: heatingat Ac₁+10° C. to Ac₁+60° C. and thereafter cooling a precursor steelpipe which contains, in mass %, C: 0.04 to 0.10% and Mn: 1.00 to 2.50%,is limited to Si: 0.80% or less, P: 0.03% or less, S: 0.01% or less, Al:0.10% or less, and N: 0.01% or less, further contains one or more of Ni:1.00% or less, Mo: 0.60% or less, Cr: 1.00% or less, and Cu: 1.00% orless, where content of Mn and content of one or more of Cr, Ni, Mo andCu satisfyMn+Cr+Ni+2Mo+Cu≧2.00, and a balance of iron and unavoidable impurities.11. A method of producing a steel pipe excellent in deformationcharacteristics according to claim 10, wherein the precursor steel pipefurther contains in mass %, one or more of Nb: 0.01 to 0.30%, Ti: 0.005to 0.03%, V: 0.30% or less, B: 0.0003 to 0.003%, Ca: 0.01% or less, andREM: 0.02% or less.
 12. A method of producing a precursor steel pipe fora steel pipe excellent in deformation characteristics according to claim10, wherein the method further comprising the steps of; heating at 1000°C. to 1270° C. a slab containing the chemical compositions claimed inclaim 10, hot rolling the heated slab to a finish rolling reduction of50% or greater; forming the obtained steel plate into an open pipeshape; and welding the seam.
 13. A method of producing a precursor steelpipe for a steel pipe excellent in deformation characteristics accordingto claim 12, wherein the slab further contains, in mass %, one or moreof Nb: 0.01 to 0.30%, Ti: 0.005 to 0.03%, V: 0.30% or less, B: 0.0003 to0.003%, Ca: 0.01% or less, and REM: 0.02% or less.